1. Field of the Invention
In amplifiers, there is the problem of occurrence of intermodulation distortion caused by the deviation of gain and phase when input power change. The present invention relates to a predistortion circuit for suppressing the occurrence of such intermodulation distortion.
2. Related Art of the Invention
As a linearizing method for suppressing the occurrence of intermodulation distortion by compensating the deviation in amplitude and phase when input power change, a predistortion scheme is known. FIG. 5 is a diagram showing the configuration and the principle of such a predisotortion scheme. First, the configuration of the predisotortion scheme shown in FIG. 5(a) is explained below. Here, an input signal is assumed to be composed of two continuous wave signals each having a different frequency component.
A signal is input from an input terminal 501, and then input to a distortion generating circuit (predistortion circuit) 502 comprising a diode, a transistor, and the like. After passing through the distortion generating circuit 502, the signal is output such as to involve a intermodulation distortion having an amplitude and phase relation so as to cancel the intermodulation distortion component to be generated by an amplifier 503 in the following stage. The signal is then input to the amplifier 503.
FIG. 5(b) shows an input signal, and FIG. 5(c) shows the spectral relation between the intermodulation distortion component of the distortion generating circuit 502 and the intermodulation distortion component of the amplifier 503. The spectrum of the input signal of the distortion generating circuit 502, consists of two basic signal frequencies f1, f2 shown in FIG. 5(b). The amplitudes of the frequencies f1, f2 in FIG. 5(c) are drawn in the same size as the amplitudes of the corresponding frequencies in FIG. 5(b) However, this is for the purpose of simplicity of description, and the actual amplitudes have been amplified.
In the spectrum shown in FIG. 5(c) of the output signal being output from the amplifier 503 to the output terminal 504, in addition to the basic signal frequencies f1, f2, there are third order intermodulation distortion components of frequencies 2f1−f2, 2f2−f1 generated by the amplifier 503, third order intermodulation distortion components of frequencies 2f1−f2, 2f2−f1 generated by the distortion generating circuit 502, fifth order intermodulation distortion components of frequencies 3f1−2f2, 3f2−2f1 generated by the amplifier 503, and fifth order intermodulation distortion components of frequencies 3f1−2f2, 3f2−2f1 generated by the distortion generating circuit 502.
As shown in FIG. 5(c), when the distortion generating circuit 502 previously generates such intermodulation distortion components as to cancel the intermodulation distortion components to be generated by the amplifier 503, an output signal is obtained without distortion in the output terminal 504.
[Problems to be Solved by the Invention]
However, for example the prior art distortion generating circuit 502 has been composed of a predistortion circuit shown in FIG. 9. Accordingly, as shown in FIG. 10, the distortion generating circuit 502 has generated signal components (f2−f1) of the differential frequencies of the input signal, 2(f2−f1) of the higher harmonics of it and at least one of the higher harmonic signal components 2f1, 2f2, (f1+f2) and so on of the carrier wave of the input signal.
These signal components (f2−f1), 2(f2−f1), 2f1, 2f2, (f1+f2) and the basic signal frequencies f1, f2 interfere with each other. This fact has been prevented the amplitude of the signal component generated by the distortion generating circuit 502 from being substantially matched with the amplitude of the signal component generated by the amplifier 503 in each of the frequencies 2 f1−f2, 2f2−f1, 3f1−2f2, 3f2−2f1 by adjusting the amplitude of the input signal and the voltage applied the circuit. Shown in FIG. 10 is the signal components output from the distortion generating circuit 502 after the interference between the signal components (f2−f1), 2(f2−f1), 2f1, 2f2, (f1+f2) and the basic signal frequencies f1, f2.
FIG. 6(a) shows an example of the basic signal frequencies f1, f2, the third order intermodulation distortion components of the frequencies 2f1−f2, 2f2−f1, and the fifth order intermodulation distortion components of the frequencies 3f1−2f2, 3f2−2f1 in the signal output from the amplifier 503.
As shown in FIG. 6(a), the amplitude of the signal component generated by the distortion generating circuit 502 is different from the amplitude of the signal component generated by the amplifier 503 in each of the third order intermodulation distortion components of the frequencies 2f1−f2, 2f2−f1. Similarly, the amplitude of the signal component generated by the distortion generating circuit 502 is different from the amplitude of the signal component generated by the amplifier 503 in each of the fifth order intermodulation distortion components of the frequencies 3f1−2f2, 3f2−2f1. Accordingly, as shown in FIG. 6(b), the third order intermodulation distortion components of the frequencies 2f1−f2, 2f2−f1 remain in the actual signal output from the amplifier 503, and so do the fifth order intermodulation distortion components of the frequencies 3f1−2f2, 3f2−2f1. As such, the distortion suppression is insufficient, and this has been a problem in the prior art.
The amplitudes of the frequencies f1, f2 in FIG. 6(b) are drawn in the same size as the amplitudes of the corresponding frequencies in FIG. 6(a). However, this is for the purpose of simplicity of description, and the actual amplitudes have been amplified.